1. Field of the Invention
The present invention relates to a method of forming a phosphor (fluorescent) layer of a gas discharge tube, and more particularly to a method of forming a phosphor layer of an elongated gas discharge tube having a diameter of approximately 0.5 to 5 mm.
2. Description of the Related Art
In a gas discharge tube such as a conventional fluorescent lamp, a phosphor layer is formed on an internal surface of the gas discharge tube by coating the internal surface of the gas discharge tube with a phosphor slurry (a coating liquid containing phosphor powder) and then drying and burning. Accordingly, the phosphor layer is uniformly formed on the internal surface of the tube. For this reason, light is emitted equally in all radial directions of the tube.
There has been known a display device in which a plurality of elongated gas discharge tubes are arranged in parallel to display images. In such a display device, a large number of discharge electrodes are provided on internal or external surfaces of the gas discharge tubes, and discharge is generated by the discharge electrodes in desired sites in the gas discharge tubes and is converted into visible light with phosphors, thereby carrying out display.
However, in the case where phosphor layers are uniformly formed on the internal surfaces of the gas discharge tubes of this display device, the phosphor layers are present on the discharge electrodes. If the phosphor layers are thus present on the discharge electrodes, the phosphors are rapidly deteriorated by the discharge. Moreover, even if electron emission layers having the effect of dropping a breakdown voltage are formed on the internal surfaces of the tubes, the phosphor layers cover the electron emission layers. Therefore, discharge characteristics are less improved and a light emission efficiency is reduced.
In addition, in the case where the phosphor layers are uniformly formed on the internal surfaces of the tubes, the visible light is emitted equally. Therefore, the efficiency of taking out emitted light toward a front surface of a screen is poor. Moreover, the expansion of discharge causes apparent expansion of pixels, which affects adjacent pixels and consequently deteriorates quality of images. Furthermore, there is a problem in that a discharge interference is caused between adjacent pixels.
Accordingly, it is desirable that the phosphor layers formed on the internal surfaces of the gas discharge tubes used in the display device should not be present on the discharge electrodes but should be formed only in positions convenient for taking out the emitted light to the front surface of the screen. However, it is hard to form the phosphor layer partially on the internal surface of an elongated gas discharge tube having a diameter of 2 mm or less and a length of 300 mm or more. Accordingly, a method of partially forming the phosphor layer has been desired.